4. Atlas and the other cervical vertebræ entirely free.
- Physalidæ.
- Agaphelidæ.
- Platanistidæ.
- Iniidæ.
- Pontoporiadæ.
- Belugidæ.
Section I. MYSTICETE (cf. p. 57).
Mysticete, Gray, Cat. Seals & Whales B. M. pp. 61, 68; Synops. Whales & Dolph. p. 1.
Mystacoceti or Balænoidea, Flower, Trans. Zool. Soc. vi. p. 110.
Head large, depressed. Teeth rudimentary; they never cut the gums. Palate with transverse, fringed, horny plates of baleen. Nostrils separate, longitudinal. Gullet very contracted. Tympanic bones simple, large, cochleate, attached to an expanding periotic bone, which forms part of the skull.
The baleen of the different Whales may be divided by its structure, by its form, and by its colour. The form and structure often go together.
The baleen consists of two parts:—1, the outer layer, called the enamel coat; and, 2, the central fibres, which form the fringe on the inner edge of the blade: both are well seen in cross sections under the microscope. The outer coat or enamel differs in thickness in the different kinds. Thus it is very thick and forms the greater part of the blade in the Greenland Whale; and in different kinds it gradually becomes thinner, until it only forms a thin coat over the central fibres. The central longitudinal fibres differ in thickness and in number. When they are very slender, as in the Greenland Whale, they form only a single layer between the two coats of enamel, and their produced ends make a very fine, long, flaccid fringe to the inner edge of the blade. In other Whales they are very numerous, in many series, and form a considerable part of the thickness of the whalebone, and make a more or less broad and rigid fringe to the blade. In some the fibres are so thick and rigid that they do not droop, but form an erect ragged edge to the short and broad blade, so rigid, indeed, that the fibres of this kind of whalebone are used to make brushes and brooms.
The whalebone varies in form, from being narrow, elongate, many times as long as it is broad at the base, by many gradations, according to the families or genera, until it is not longer than broad. The longest blades have the most enamel and the finest and most flaccid fibres, which, on the other hand, gradually (as it belongs to different genera) become coarser and more rigid as the whalebone diminishes in length compared with its breadth.